The invention pertains to a seal in a high voltage compound insulator which is comprised of a glass fiber reinforced synthetic plastic rod with a surrounding coating and suspension mounting located at the ends of the rod, wherein a part of each suspension mounting surrounds the coating of the rod and each suspension mounting is provided with a circumferential bead pointing outwardly in the radial direction.
Difficulties have arisen in connection with the aforementioned type of insulators, which insulators are described, for example, in U.S. Pat. No. 4,246,696. Specifically, chemicals dissolved by humidity have penetrated the inside of the insulator rod, thereby initiating electrical and mechanical destruction therein. Penetration occurs preferentially at the interface of two different materials, such as, for example, at the boundary surface between the glass fiber reinforced synthetic plastic rod and the metal suspension mounting. Attempts have therefore been made in the art to seal these critical locations by fitting the surrounding coating of the glass fiber reinforced rod over the suspension mounting, thereby using it as a seal. Such a measure is disclosed in U.S. Pat. No. 3,898,372. However, actual practice has shown that seals of this type are vulnerable to breakdown, because the electric potential migrates over the atmospherically soiled insulator surface to the other mounting.
A second approach has been described in U.S. Pat. No. 4,281,943, wherein the glass fiber reinforced rod is projected with its surrounding coating into a metal mounting equipped with a circumferential bead at the end of the mounting which faces the insulator. It is the function of this bead to render unstable partial arcs on the insulator, thereby preventing heat damage to the insulator rod. But there remains a gap between the inner surface of the mounting and the coating, into which corrosive atmospheric components may penetrate.
Sealing measures of the type proposed in the preceding paragraph have the disadvantage that they become ineffective in time, when significant temperature fluctuations outdoors of more than 100.degree. C. cause the synthetic plastic materials to give way under the applied pressure. Thus, they do not assume quite their original shape when the pressure is relieved, if an attempt is made, as set forth in U.S. Pat. No. 4,427,843, to press the part of the mounting surrounding the coating radially onto the coating in order to obtain airtightness. If it cannot be ensured that this contact pressure is not reduced, after a certain number of temperature cycles, gaps will appear, especially between the suspension mounting and the insulator rod, even if the gap is filled with known, hardenable polymers such as epoxy resins or silicone rubber.
A further measure to prevent the formation of gaps is disclosed in Austrian Pat. No. 365 846, whereby the space between the suspension mounting and the coating is filled with a synthetic grease such as silicone grease. These synthetic greases, in particular silicone grease, are composed of organic chain compounds. Their thixotropy is adjusted by means of mineral or organic fillers, such as, for example, very fine grained silica in silicone grease. However, the low viscosity constituents of these synthetic greases tend over time to diffuse into the adjacent material, e.g., the glass fiber reinforced rod of the insulator discussed above. A filler skeleton with the high viscosity organic components remains. Furthermore, experiments at temperatures around 100.degree. C. and for extended exposure times have shown that these synthetic greases form fine internal cracks. This is readily demonstrated by exposing the insulator to various temperature cycles and then immersing it in a water bath colored with a red dye, followed by an inspection inside for traces of the dye. It is also difficult to place such greases into small gaps so that all cavities are filled exactly, since the greases must be highly viscous to keep them from running out of the gap at elevated temperatures and over extended periods of time.